Modulators are an essential part of adaptive signal cancellers. The function of a modulator is to provide a signal exactly in amplitude but 180 degrees out of phase with an unwanted signal existing in the canceller circuit. The result is that the unwanted signal is nulled out and cancelled. Unwanted signals may be, for example, signals from a nearby transmitter coupled into the input circuits of a receiver, or distortion causing signals in transmitter circuits.
In fulfilling its function the modulator accepts as an input a carrier signal which it proceeds to vary in amplitude and phase, under control of a modulating signal. The variation in amplitude is over a range from zero to some maximum value and in phase from 0 degrees to 360 degrees. Also, to minimize the generation of other unwanted signals, the distortion content of the modulated carrier signals at the modulator output must be kept low. The distortion products produced by the modulator are grouped around harmonics of the carrier signal and consist of harmonics of the carrier and modulating sidebands plus intermodulating products between all these signals. Adaptive cancellers are important to the operation of the feed forward cancellation circuits used to linearize high frequency transmitters. Modulators that can independently vary the amplitude and phase of a carrier perform this function by combining together two signals in phase quadrature. This is shown in FIG. 1 where V(x) and V(y) represent the amplitudes of two signals formed in quadrature from the carrier. These are combined together to yield a resultant V(r). By independently varying the amplitude of the two signals V(x) and V(y), the amplitude of the resultant V(r) can be varied from zero to a maximum, and the phase from 0 degrees to 90 degrees, that is within the quadrant bounded by V(x) and V(y). However by reversing the polarity of either or both V(x) and V(y), the phase of the resultant can be made to assume a value in the other 3 quadrants and, of course, the amplitude can be varied as before. Typically, control of the amplitude and sign of either of the two components V(x) and V(y) has been by reflection within a hybrid. For this purpose a quadrature hybrid is most convenient. As shown in FIG. 2 the carrier amplitude V is fed into port 1 and the output appears at port 4. The amplitude of the output can be varied between V and -V by varying the value R, of the two equal resistors terminating ports 2 and 3. With R=Z(0) the characteristics impedance of the hybrid, all he power incident on port 1 is absorbed by the two terminating resistors at ports 2 and 3 and the output at port 4 is zero. With R=0 all the power is reflected from ports 2 and 3 to port 4 with a phase -90 degrees with respect to the input carrier (neglecting the insertion phase of the hybrid) and an amplitude V. With R=infinity, again all the power is reflected from ports 2 and 3 to port 4 but with a phase -270 degrees. The amplitude of this signal is also V as before but with a phase difference of -180 degrees compared to the previous phase condition, so it is now denoted by "V. By varying R between 0 and infinity the amplitude of the output signal may be varied between the limits V through 0 to -V. In effect, the combination of the hybrid and the variable resistors forms an amplitude modulator. A quadrature hybrid is the most convenient reflective device as equal resistors can be used at ports 2 and 3. An ordinary hybrid requires different resistors at these two ports to produce the same effect.
With two such amplitude modulators and an extra quadrature hybrid to provide two signals in phase quadrature a complete phase and amplitude modulator can be constructed as shown in FIG. 3. Quadrature combination is used with amplitude modulation of quadrature components V(r) #1 and V(r) #2 and the resultant modulated signal can be varied in phase from 0 degrees to 360 degrees and in amplitude 0 to V.
In common practice, the combination of a pair of variable resistors and a reflective hybrid has formed the heart of all modulators. The variable resistors have been provided by a collection of diode-resistor branches which are turned on or off sequentially to gradually adjust the network resistance. The resistors in each branch provide incremental adjustment of the resistance of the network, as their companion diodes are turned completely on or off. Between these resistance steps the resistance of the network is continuously adjusted by varying the current through the branch that is partially on.
This method suffers from two disadvantages. First the diode-resistor branch that is partially on generates a high level of harmonic distortion. To reduce distortion, more branches with more diodes can be added, in effect sharing signal power between more diodes. However, this increases circuit complexity.
Second, as a result of stepping through a number of diode-resistor branches to reach a particular value of network resistance, a number of diodes are turned on. Each diode generates distortion as it turns on and this results in a sequence of bursts of distortion being generated.
Accordingly, it is one object of this invention to provide a phase and amplitude modulator which does not involve the use of noisy diode-resistor networks.
It is a further object of this invention to provide a modulator that is of efficient and non-complex design.